1. Field of the Invention
The present invention relates to a magneto-optical information storage medium comprising a bias layer for applying a bias magnetic field, which is necessary for recording and erasing information on/from a recording layer, to the recording layer, and a method for recording and erasing information thereto and therefrom.
2. Description of the Related Art
In a magneto-optical recording system, information is recorded/reproduced by using an information storage medium comprising a recording layer formed of a magnetic film having an axis of easy magnetization in a direction perpendicular to its surface. According to this system, information is recorded by reversing the magnetization upon radiation of a laser beam and application of a magnetic field. The recorded information is reproduced by detecting the direction of the magnetization of the recording layer utilizing a magneto-optical effect such as the polar Kerr effect. In comparison with a normal magnetic recording system, the magneto-optical recording system is superior in non-contact accessability and compatibility of media. In addition, the magneto-optical recording system can increase a recording density higher than the magnetic recording system. The magneto-optical recording system, however, has a limitation based on the principle of the system itself. That is, when recorded information is to be rewritten, the previously recorded information must be erased first. For this reason, in the magneto-optical recording system, it is considered that direct information rewriting by overwrite is difficult.
Recently, various proposals have been made in order to enable direct overwrite in a magneto-optical recording system. For example, U.S. patent application Ser. No. 649,519, discloses a system wherein a magneto-optical information storage medium comprises a temperature-dependent bias layer adjacent to a recording layer so that magnetization reversal of the recording layer is performed in a reversible manner by using a bias magnetic field from the bias layer.
The bias layer described in this patent has a compensation temperature lower than the Curie temperature of the recording layer, and has a Curie temperature higher than the Curie temperature of the recording layer. In this magneto-optical information storage medium, the directions of magnetization of the recording and bias layers are the same, e.g., a downward direction prior to recording (after initialization or erasing).
When a laser beam is radiated onto the recording layer in this state, thus the temperature of a magnetic domain where the laser beam is radiated reaches the Curie temperature, the magnetization of the magnetic domain is eliminated. Then, if the temperature of a magnetic domain of a bias layer corresponding to this magnetic domain exceeds the compensation temperature, the direction of the magnetization of the magnetic domain of the above-described recording layer is reversed upward.
After this operation, when radiation of the laser beam is stopped, the recording and bias layers are cooled, and first, the magnetic domain of the recording layer is magnetized in the direction opposite to that before recording upon magnetization of the corresponding magnetic domain of the bias layer (i.e., upward). Subsequently, when the temperature of the magnetic domain of the bias layer becomes lower than the compensation temperature with a decrease in temperature of the recording and bias layers, the magnetization of the magnetic domain of the bias layer is temporarily reversed downward. However, since the bias layer is formed of a material exhibiting a sufficiently small coercive force near room temperature, its magnetization is reversed upward by magnetization of the recording layer.
When recorded information is to be erased, a laser beam is radiated onto only a magnetic domain where the information is recorded. The magnetization of the magnetic domain of the bias layer is reversed downward by heating the magnetic domain of the recording layer to the neighborhood of the Curie temperature. Then, when radiation of the erase laser beam is stopped, the magnetization of the magnetic domain of the recording layer is reversed downward by magnetization of the corresponding magnetic domain of the bias layer.
In this method, the magnetization of the recording layer is reversed by a laser beam having the same energy in both recording and erasing operations. For this reason, in the erasing operation, the magnetization of the bias layer must be directed to its original direction by radiating only a magnetic domain where information is recorded. Therefore, when overwrite is to be performed, previously recorded information is read, and then an overwrite beam must be radiated on the basis of the readout information.
In order to perform such control, two independent laser beams must be used such that while a magnetic domain where the information is previously recorded is detected by using a first laser beam, a second laser beam for recording/erasing is controlled.
However, it is difficult to perform such control with high precision. In addition, when such control is to be performed, an information erasing operation becomes extremely complicated. Two systems are employed as those using two laser beams, i.e., a two-head/two-beam system wherein two beams are generated by independent semiconductor lasers and a one-head/two-beam system wherein two beams are generated by one semiconductor laser. The former has a problem of high cost, whereas the latter has problems that the manufacture of a semiconductor laser as a laser beam source is difficult and the arrangement of its optical system is complicated.
A technique of realizing high-speed overwrite by using a one-head/one-beam system without posing the above-described problems is disclosed in Extended abstract (The 34th Spring meeting 1987), The Japan Society of Applied Physics and related societies; Saito et al. 28p-ZL-3, and Japanese Patent Disclosure (Kokai) No. 62-175948. According to this technique, a permanent magnet for generating a magnetizing strong magnetic field is arranged at a previous portion of a radiation position of a laser beam, and high-speed overwrite of a two-layered magneto-optical information storage medium is performed by power modulation of light beam.
In this method, however, since a large permanent magnet for generating a strong magnetic field is used, the size of an apparatus is inevitably increased, and besides, an electromagnetic drive system for tracking and focusing in an optical head is adversely influenced by leaking magnetic fields of the magnet.